The importance of electrically conductive polymer as one of fields of functional polymer has been gradually increased. By providing electrical conductivity to a polymer material, the polymer material obtains useful physical and chemical properties and not only a functionally superior material, but also a cheap material in view of production costs can be obtained.
In general, a number of polymer materials have been regarded as highly insulating materials. Although the polymer materials work well as electrically insulating materials due to a low conductivity, they function as electrical conductors when a filler such as carbon black, carbon fiber, or metal powder is added.
The added filler forms an electrical path in the polymer material which works as a passage of electrons so that the polymer material becomes an electrical conductor. When the temperature increases, the interval between filler particles in semicrystalline polymer including the conductive filler increases due to a thermal expansion in a melting area of the polymer so that the flow of electrons is disturbed.
Carbon black and carbon fiber are mainly used as the conductive filler added to provide a positive temperature coefficient (PTC) function to the polymer. Crystalline polymer such as polyethylene is mainly used as the polymer material.
Accordingly, as the temperature increases, the resistance of the polymer material is suddenly increased greatly, which is referred to as a static characteristic temperature coefficient or a PTC phenomenon. That is, while resistance is relatively low at a low temperature, when the temperature reaches a predetermined degree, the resistance increases suddenly so that current is difficult to flow. The temperature at which the above sudden change occurs is referred to as a switching temperature or Curie temperature.
The switching temperature is defined as a temperature corresponding to double the minimum resistance value or a resistance value at a reference temperature (25° C.) and is a major parameter in the property of the material.
Also, changing the component of the material makes the switching temperature move toward a high temperature or a low temperature so that the material can be used for a variety of devices. For example, the material can be used for a temperature sensor or overheat protection using a resistance-temperature property, a heater using a current-voltage property, or a delay circuit or a demagnetic circuit using a current attenuation property.
Of the above application fields, in the case of being used to prevent a damage to a product or an electronic circuit due to overheat or the flow of over-current, the PTC using polymer can greatly perform both protection functions with respect to overheat and overload.
For a fuse used as an overload protection, although it has a superior protection function with respect to over-current, when current is discontinued as the fuse is cut off due to the over-current, the fuse needs to be replaced, so it is inconvenient. For a bimetal switch which provides a superior temperature protection function and a restoring function, since it is not sensitive to over-charges, it is difficult to use the bimetal switch for a precise electronic circuit. Thus, it can be seen that the PTC using polymer has a superior property compared to the above members.
The polymer PTC material can be used as a superior PTC material by compensating for drawbacks of a conventional ceramic PTC such as a low conductivity, high process costs, and a fixed shape. In particular, since the minimum resistance is quite small and a manufacturing shape is free, the polymer PTC material has already been widely used in designing small devices and the use thereof is fast increasing. The temperature of the polymer PTC decreases after heat or current is cut off. Also, the PTC material has a function of automatically restoring without being replaced when the over-current is removed.
In addition to the above properties of the PTC, a negative temperature coefficient (NTC) phenomenon occurs in which resistance decreases greatly when a new conductivity network is formed as the dispersion state of conductive particles in a melting state of polymer changes.
Since the property provided to the conductive polymer by the PTC effect can be lost by the NTC phenomenon, the NTC phenomenon becomes a great hindrance to the PTC phenomenon.
The NTC phenomenon occurs when the conductive particles are moved by cross-linking in a melting state so that a new structure is formed. The cross-linking forms a network to allow the conductive particles to strongly attract to each other and restrict motion of the conductive particles so that a structural stability can be obtained.
The polymer PTC material is used to prevent damage to electronic products or electronic circuits and has already been used in designing small devices because the manufacturing shape thereof is free. However, since a cross-linker is added to restrict the NTC phenomenon and then the polymer PTC material is cured so that it has a hard plastic structure, the polymer PTC material has a limit in the process and purpose thereof when being used for a general heating body.
In the semicrystalline polymer including a conductive filler, as the temperature increases, since the interval between filler particles in the polymer increases accordingly due to thermal expansion in the switching temperature area, an amplitude between thermal contraction and thermal expansion that repeat, continuously occurs up to a crystalline melting point so that the life span of products are shortened.